Conventionally, for venous blood collection, a tourniquet may be placed tightly around some portion of an extremity, typically between the subject's heart and the location from which the blood sample is collected or drawn. For example, when drawing blood from the subject's forearm, a tourniquet (e.g., an elastic band) may be placed around the subject's upper arm. The tourniquet restricts the flow of blood to the sampling or drawing location and also makes the veins inside the subject's elbow more pronounced and easier to find and to puncture with a needle.
A similar device does not exist for capillary blood collection, for example from the subject's finger. Typically, finger tourniquets have been used to stop blood flow entirely but not to restrict the flow of blood to the end of the digit where a blood sample is usually collected. Very small volumes of blood (e.g., about 150 μl) may be drawn without using a tourniquet (e.g., using a capillary pipette), but larger volumes of blood are generally not sampled from a subject's fingertip.
Taking a blood sample from a subject's finger using capillary blood collection techniques can also be challenging due to dozens of variables that might reduce capillary blood flow. For example, dehydration, fatigue, lack of exercise, cold weather, and cold hands may reduce blood flow to the collection site. Circulatory disorders, resulting from, for example, obesity, diabetes mellitus, arthritis, disability, heart conditions, and arterial issues, may also cause reduced blood flow to peripheral regions, such as the subject's hands or feet.